The youngest son of the North Korean leader, Kim Jong-il, became a member of parliament last year, South Korean news reports said today.

The reports are another sign that Kim Jong-un is being groomed to succeed his father as the head of the communist regime.

It is widely believed that 68-year-old Kim plans to hand power to his third son at some point, although little is known about him.

Speculation about the North's succession plans has intensified since the senior Kim reportedly had a stroke in 2008. He has led North Korea since 1994, when he took power in a hereditary succession.

Kim Jong-un reportedly ran for a parliamentary seat in elections last March. His name, however, was not on the list of Supreme People's Assembly legislators, sparking speculation that he may not have run in the polls or could have used an alias.

On Tuesday, the mass-circulation Dong-a Ilbo newspaper quoted a high-level western source as saying Kim was elected in the rubber-stamp legislature's Constituency No 216.

The source said he obtained the information from unidentified North Korean figures about two months after the vote.

Another national daily, JoongAng Ilbo, carried a similar report, saying the comments would confirm that Kim had formally entered government service after being touted as the North's next leader.

The paper also said Constituency No 216 has a special meaning in North Korea because Kim Jong-il's birthday falls on 16 February. It said the western source spoke during a meeting with journalists in Seoul yesterday.

A spokesman at South Korea's main spy agency, the National Intelligence Service, said his organisation believed there was a low probability the younger Kim had become a member of parliament.

The spokesman declined to elaborate on the agency's opinion and spoke on condition of anonymity.

North Korean leaders hold absolute power in the impoverished country, which has active nuclear and missile programmes.

The National Intelligence Service chief, Won Sei-hoon, told MPs last week that North Korea had launched a propaganda campaign aimed at making its 24 million people adore Kim Jong-un, including releasing songs and poems praising him.

On Saturday, North Korea said it would hold a rare Workers' party conference in September to choose new leaders – a move experts say may be aimed at giving Kim Jong-un a top party job.

Tension has been high on the Korean peninsula over the sinking of a South Korean warship in March, blamed by the South on Pyongyang.